The invention relates to a process for the direct hydrogenation of halogen-substituted silicon compounds. Hydrogen-substituted silicon compounds have achieved particular industrial importance. Thus, monosilane (SiH4) is used for producing high-purity silicon for semiconductor technology and, like disilane (Si2H6), for the (epitaxial) deposition of thin layers in microelectronics in the production of chips and thin-film solar cells. Moreover, in the preparation of organofunctional (poly)siloxanes, hydrogen-substituted organochlorosilanes, for example (CH3)2SiHCl, (CH3)SiH2Cl, (CH3)SiHCl2, etc., which are obtained only in insignificant amounts or in amounts which do not correspond to the respective demand in the industrial Mxc3xcller-Rochow direct synthesis, are key compounds for the known hydrosilylation reaction.
There has therefore been no lack of attempts to produce the various silanes on an industrial scale. Neither the acidolysis of silicides nor the reaction of silicon tetrachloride with lithium alanate in ether solution are suitable for producing silanes inexpensively and in the required purity. The catalytic disproportionation of trichlorosilane carried out on a relatively large scale forms not only one part of monosilane but also three parts of silicon tetrachloride as by-product which has to be worked up. Finally, alanates or amine-alane adducts (R3Nxe2x80x94AlH3) prepared therefrom are reacted with silicon tetrafluoride in organic solvents to form monosilane.
A solvent melt as reaction medium has been used in a continuous or cyclic process and developed for industrial use. In this process, silicon tetrachloride is hydrogenated by means of lithium hydride which is prepared from lithium in an LiCl-KCl melt at 400xc2x0 C., dissolved in the process and subsequently reacted. Disadvantages are the high temperature, corrosion problems, the high price of lithium compounds and the environmental concerns associated with them, and finally the reprocessing cost for the lithium chloride formed [(1) DE-C 1080077; (2) W. Sundermeyer et al. Angew. Chem 70 (1958) 625].
The direct hydrogenation of silicon tetrachloride in a eutectic mixture of sodium chloride and aluminum chloride (m.p. 108xc2x0 C.) is described by H. L. Jackson et al. [Inorg. Chem. 2 (1963) 43]. In the reaction of hydrogen with a high molar excess of aluminum powder, 30 g of silicon tetrachloride are hydrogenated at 175xc2x0 and an industrially impractical pressure of 960 bar in a 400 ml shaking autoclave charged with steel balls for 6 hours to give a conversion of 70-100% and 5.8 g of monosilane. Disilane and dimethylsilane are formed only in very small amounts under analogous reaction conditions because of cleavage of the Sixe2x80x94Si or Sixe2x80x94C bonds. Monosilane is also formed only in traces when the aluminum is not additionally treated with up to 3% of an xe2x80x9cactivatorxe2x80x9d such as lithium hydride, lithium alanate, sodium hydride, alkaline earth metal hydrides and the abovementioned melt. The reaction mechanism is said to be the formation of HxAlCl3xe2x88x92x as hydrogenating agent.
The analogous reaction of (organo)chlorosilanes with hydrogen and aluminum in salt melts comprising sodium chloride and aluminum chloride is described by H. J. Klockner et al. [EP 0412342] as proceeding under atmospheric pressure when the aluminum contains 0.03-0.25% by weight of a hydrogen-transferring metal such as titanium, zirconium, hafnium, vanadium, niobium or nickel, according to the examples as alloy. In addition, this document also claims, inter alia, the same xe2x80x9cactivatorsxe2x80x9d as those described by H. L. Jackson et al.: Lithium alanate and sodium hydride, and additionally titanium hydride. In another document [DE 4119578.7], lithium hydride (cf. H. L. Jackson et al.) and also zirconium hydride, palladium chloride, nickel chloride and Raney nickel are claimed as xe2x80x9cactivatorsxe2x80x9d. As regards the reaction mechanism, the generation of HxAlCl3xe2x88x92x as actual hydrogenating compound which is said to be present in a concentration of 0.01-20 mol %, in particular 10 mol %, based on the amount of melt, is once again described. When using a melt having a molar ratio of NaCl:AlCl3=1:1, 90% yields of SiH4 and 83-90% yields of (CH3)2SiH2 were observed. However, the conversions (% of the total starting compound used which is reacted) and, even more, the space-time yields (STY; g/l of reaction volume and hour) of the individual components are extraordinarily low. In the case of SiCl4, the conversion is 20% (66 g of 324 g in 6 h) and the STY is only 1.9 g of SiH4/lh. (CH3)2SiH2 was able to be obtained in a conversion of only 5-8% (15.5 g of 327 g in 2 h) and an STY of 3-6 g/l h. In each case, only up to about 3% of the hydrogen introduced was reacted, since according to the method described it has to be used in a large, 5-10-fold excess.
It is therefore an object of the invention to develop a process for the direct hydrogenation of Sixe2x80x94Cl bonds using elemental hydrogen at moderate temperatures and pressures, which process avoids the stated disadvantages and makes it possible to prepare silanes in a technically simple and economical manner.
Surprisingly, metals which form interstitial hydrides, preferably titanium, zirconium, vanadium, chromium, manganese, nickel, palladium, platinum and rare earth metals, or combinations thereof, in particular titanium metal, have now been found to be particularly good hydrogen transferrers for achieving this object when they are present in suspension in very finely divided form, preferably when they are produced in situ in a salt melt comprising alkali metal/alkaline earth metal halides and aluminum halide, in particular the chlorides, by reduction of the corresponding metal halide by an electropositive element (halogen acceptor I). As metal halides, it is possible to use all solid or volatile compounds, for example in the case of titanium the compounds of the type TiX4, TiX3, TiX2, or salts containing TiX62xe2x88x92 or TiX63xe2x88x92 ions (X=halogen), but preferably TiCl4 or TiCI3, which are dissolved in the melt and reduced to titanium metal by the halogen acceptors I, namely magnesium, calcium or aluminum. The so called xe2x80x9cactivatorsxe2x80x9d in the form of various previously prepared, expensive hydrides or alloys as are known from the prior art are not necessary in the process of the invention.
The salt melt serving as reaction medium comprises mixtures of alkali metal halides and/or alkaline earth metal halides and aluminum halides, preferably the chlorides. The liquidus curves of the individual systems can be found in standard tables [R. S. Roth, M. A. Clevinger, D. McKenna, Phase Diagrams for Ceramists, National Bureau of Standards, The American Ceramic Society, Inc., Vol. I-V, 1964-83]. The eutectics or melts even richer in AlCl3 can be used, but they suffer from a high degree of AlC3 sublimation, which would require appropriate, technically complicated measures in carrying out the reaction.
It has therefore been found to be advantageous to use the (tetra)chloroaluminates whose vapor pressure becomes significant only far above the reaction temperatures to be employed according to the invention.
Examples are:
It has now surprisingly been found that pseudobinary and pseudoternary mixtures of these chloroaluminates are, owing to the favorable change in physical properties of the melt associated therewith, particularly advantageous for the preparation of and the direct hydrogenation using finely dispersed metals forming interstitial hydrides, for example the eutectics:
Preference is given to using the system NaAlCl4xe2x80x94KAlCl4 in a molar ratio of the chloroaluminates of 70:30 (eutectic), advantageously with a further addition of LiAlCl4, e.g. NaAlCl4xe2x80x94KAlCl4xe2x80x94LiAlCl4 in a molar ratio of 70:25:5.
The reactions are carried out at above the melting point of the chloroaluminate system used in the particular case and at below the decomposition temperature of the starting materials to be hydrogenated and of the products, i.e. in the range from room temperature to 600xc2x0 C., preferably in the range 100-400xc2x0 C., in particular in the range 150-300xc2x0 C.
The halogen acceptor I suspended in an at least stoichiometric amount in the chloroaluminate melt by intensive stirring reduces the metal halide added to the metal. The reduction can be carried out under inert gas, but preferably already in a hydrogen atmosphere.
The subsequent hydrogenation of the halogen-substituted silicon compounds in the presence of the metal forming interstitial hydrides is carried out by passing them continuously together with hydrogen into the salt melt with addition of a halogen acceptor II in an at least stoichiometric amount based on the chlorosilane used. For further hydrogenations in the same melt, it is only necessary to add further halogen acceptor II in an amount corresponding to that which has been consumed. An advantage which has been found is that the same electropositive elements can be used as halogen acceptors I and II with the choice of electropositive elements depending on the melt system used in each case.
As regards the particle size of the added halogen acceptors I and II, there is no need to use fine powder grades which are particularly difficult to produce and handle. Even if these can be used in principle in the reaction according to the invention, preference is given to powders of, for example, ordinary smelter aluminum having a particle size range of 150-1000 xcexcm. Even larger particle sizes are likewise suitable, as long as they do not have an adverse effect on the stirrers of the reactor.
The hydrogenation is normally carried out at atmospheric pressure. It can also be carried out at superatmospheric pressures to increase the hydrogen solubility, although a similar effect can also be achieved by dispersion by means of suitable, preferably sparging, stirrer types. These offer the additional advantage that always only the hydrogen consumed has to be replaced in a stoichiometric amount, based on the halosilane to be hydrogenated, while the passage of a large excess of hydrogen, which can in principle also be employed in the process of the invention, is disadvantageous in terms of its recovery or entrainment of product. The silanes are usually separated off by condensation in cold traps and are subsequently purified by low-temperature rectification [H. Hiller, gas aktuell (Messer Griesheim), 33, 1987, 2-6]. At high throughputs of the hydrogen/halosilane mixture, partially hydrogenated (organo)chlorosilanes were also able to be obtained as by-products using the process of the invention.
The addition of an excess of dissolved and/or suspended alkali metal halide or alkaline earth metal halide (chloride) has been found to have a particularly good promoting effect on the reaction in the hydrogenation of the invention; this additional halide is preferably used in such a mixing ratio that the liquidity of the salt melt is not significantly changed, if at all. The continual increase in the volume of the melt caused by the reaction by-products is countered by occasionally draining or siphoning off part of the melt and working it up separately. The use of a salt melt as reaction medium also. makes it possible, as a combination step, to recover the halogen acceptor metals by electrolysis directly in the salt melt using methods known from the literature [H.J. Klockner et al., Z. anorg. allg. Chem. 509 (1984) 76-84], advantageously also using a self-dissolving silicon anode and with simultaneous production of silicon tetrachloride [W. Sundermeyer et al., Chem. Ing. Techn. 37, (1965) 14].
The following examples demonstrate that silanes can be produced in high yields and at conversions and space-time yields far superior to the prior art by means of the process of the invention.